1. Field of the Invention
The present invention relates to a semiconductor device, in particular, to a boost-type DC-DC converter which is provided with a boost converter circuit for increasing an input voltage to a predetermined voltage and outputting the predetermined voltage.
2. Description of the Prior Art
As such a type of a DC-DC converter (switching power-supply circuit) in the prior art, there is one disclosed in Japanese Laid-Open Utility Model Application No. 6-60291 (see FIG. 1).
In a switching power-supply circuit shown in FIG. 1, a capacitor C3 is connected between an input terminal 1 and a power-source-voltage terminal Vcc of a control circuit 3 which drives a switching element (transistor) Q1. When the switching power-supply circuit is started, a DC power is supplied to the switching power-supply circuit externally, and the power is supplied to the control circuit 3 via the capacitor C3.
Further, a capacitor C1 is connected between the input terminal 1, which is connected to an external DC power source, and ground. Furthermore, the input terminal 1 is connected to an end of a primary winding N1 of a transformer T. The other end of the primary winding N1 of the transformer T is connected to the ground via the drain and source terminals of the MOSFET-type switching transistor Q1. The gate of the switching transistor Q1 is connected to the output terminal of the control circuit 3.
The capacitor C3 and a diode D2 are connected in parallel between the power-source-voltage terminal Vcc of the control circuit 3 and the input terminal 1, and the GND terminal of the control circuit 3 is connected to the ground. The anode of the diode D2 is connected to the input terminal 1 and the cathode of the diode D2 is connected to the control circuit 3. One end of a secondary winding N2 of the transformer T is connected to the anode of a diode D1, and the other end of this winding N2 is connected to the ground. The cathode of the diode D1 is connected to an output terminal 2. A smoothing capacitor C2 is connected between the output terminal 2 (output voltage V.sub.O) and the ground. Further, in parallel to the smoothing capacitor C2, a series circuit of resistors R1 and R2 is connected. The connection point between the resistors R1 and R2 is connected to the control circuit 3. Thereby, the output voltage is divided by the resistors R1 and R2, and the thus-obtained voltage is input to the control circuit 3.
One end of a tertiary winding N3 of the transformer T is connected to the ground, and the other end of this winding N3 is connected to the anode of the diode D3. The polarity of the end of the primary winding N1 of the transformer T on the switching-transistor-Q1-drain side is the same as the polarity of the end of the secondary winding N2 of the transformer T on the diode-D1-anode side. The cathode of the diode D3 is connected to the power-source-voltage terminal Vcc of the control circuit 3.
In the switching power-supply circuit, the power-source voltage for driving is provided to the control circuit 3. At the time of starting of the switching power-supply circuit, when the input voltage V.sub.IN is applied to the input terminal 1 by the external DC power source, a current i.sub.C3 instantly flows in a path from the input terminal 1, through the capacitor C3, through the control circuit 3 and to the ground, and, thereby, the capacitor C3 is charged. Due to the property of capacitors, in a condition in which the capacitor C3 was not previously charged, immediately after the input voltage V.sub.IN is applied, the electric potential difference between the ends of the capacitor C3 is zero. Then, an increasing electric potential difference between the ends of the capacitor C3 develops by the charging current i.sub.C3 as time elapses. Thereby, immediately after the input voltage VIN is applied, a voltage approximately equal to the input voltage V.sub.IN is provided to the power-source-voltage terminal Vcc of the control circuit 3.
The control circuit 3 is started by this input voltage V.sub.IN, and a pulse voltage having an amplitude approximately the same as the voltage V.sub.IN is applied to the gate of the switching transistor Q1. Thereby, the switching transistor Q1 operates, and the switching power-supply circuit starts operating. Therefore, it is possible to set the voltage value of the necessary lowest input voltage V.sub.IN by which the switching power-supply circuit can be started to a value approximately near to the threshold value (V.sub.TH) of the switching transistor Q1. Therefore, in comparison to a conventional circuit, it is possible to set the voltage value of the necessary lowest input voltage V.sub.IN to a lower value.
When the switching power supply circuit starts operating, a voltage is induced in the secondary winding N2 of the transformer T, DC power is supplied to a load, and, simultaneously, a voltage V.sub.3 is also induced in the tertiary winding N3 of the transformer T. When the voltage V.sub.3 is in a relationship such that (V.sub.3 -V.sub.F3)&gt;(V.sub.IN -V.sub.F2) wherein V.sub.F2 stands for the forward-direction dropping voltage of the diode D2 and V.sub.F3 stands for the forward-direction dropping voltage of the diode D3, the voltage V.sub.3 induced in the tertiary winding N3 of the transformer T is rectified by the diode D3 (the forward-direction dropping voltage: V.sub.F3), then, is smoothed by the capacitors C3 and C1, and is input to the power-source-voltage terminal Vcc as an output voltage Vs (=V.sub.3 -V.sub.F3) of an auxiliary power-source circuit 4.
Thereby, power (voltage=V.sub.3 -V.sub.F3) is supplied to the control circuit 3 from the auxiliary power-source circuit 4. The voltage value of this voltage Vs is higher than the input voltage V.sub.IN. Therefore, a voltage higher than the voltage at the time of starting is applied to the gate of the switching transistor Q1, which is driven thereby. Thereby, the ON resistance of the switching transistor Q1 is smaller than that at the starting. As a result, in a steady operation after the starting, the power consumption in the switching transistor Q1 is smaller than that at the starting.
When the input voltage V.sub.IN increases and there is a relationship such as (V.sub.IN -V.sub.F2)&gt;(V.sub.3 -V.sub.F3), power of the input voltage V.sub.IN from the external DC power source is supplied to the control circuit 3. Thus, the higher voltage between V.sub.IN and V.sub.3 is automatically selected as the driving power source for the control circuit 3.
In this switching power-supply circuit in the prior art, as described above, when the voltage V.sub.3 is in the relationship such as (V.sub.3 -V.sub.F3)&gt;(V.sub.IN V.sub.F2), the voltage V.sub.3 induced in the tertiary winding N3 of the transformer T is rectified by the diode D3, and, then, is smoothed by the capacitors C3 and C1. Thus, the voltage having the magnitude of V.sub.3 -V.sub.F3 is input to the power-source-voltage terminal Vcc as the output voltage Vs of the auxiliary power-source circuit 4. When there is the relationship such as (V.sub.IN -V.sub.F2)&gt;(V.sub.3 -V.sub.F3) before the step-up (boost) operation, because the relationship such as (V.sub.IN V.sub.F2)&gt;(V.sub.3 -V.sub.F3) holds, the voltage V.sub.IN -V.sub.F2 which is smaller than the input voltage V.sub.IN from the external DC power source by V.sub.F2 is provided to the control circuit 3.
However, in the switching power-supply circuit, in order to lower the power-source voltage, it is necessary to lower the necessary lowest operation voltage of the control circuit 3 because the voltage provided to the power-source-voltage terminal Vcc is lower than the input voltage V.sub.IN by V.sub.F2. However, because the operation-possible lowest voltage of the MOS transistors constituting the control circuit 3 is limited, it is not possible to lower the necessary lowest operation voltage of the control circuit 3 without limit. Therefore, in order to lower the power-source voltage, it is desired to omit the voltage drop developing through the forward-direction dropping voltage of the diode. However, in the switching power-supply circuit in the prior art, it is difficult to omit the voltage drop developing through the forward-direction dropping voltage of the diode.